A new assay for the determination of faecal alpha-1-antitrypsin including a simplified extraction of native stool and nephelometric measurement was studied. Its validity was established by a comparison with standardized methods and with well-known activity parameters. Excellent correlations were found, comparing this method with radial immuno-diffusion carried out on lyophilized (R = 0.96; p less than 0.05) and native (R = 0.97; p less than 0.05) stool samples. Faecal alpha-1-antitrypsin concentrations as measured with this method were significantly (p less than 0.001) higher in patients with inflammatory bowel diseases (Crohn's disease = 40, ulcerative colitis = 15) than in 25 normal controls (0.51 +/- 0.06 mg/g vs. 0.13 +/- 0.02 mg/g; x +/- SEM). There was a significant correlation of faecal alpha-1-antitrypsin with CDAI, activity index van Hees, and with various laboratory parameters (ESR, CRP, serum alpha-1-antitrypsin, orosomucoid, albumin, iron, haematocrit, haemoglobin, leucocytes, and thrombocytes). The presented method is equivalent to standard techniques in measuring faecal alpha-1-antitrypsin concentrations. It is highly useful for clinical routine and for follow-up studies in patients with inflammatory bowel disease.